What is an Aggressive Cam?
When you hear that a cam's profile is aggressive or radical, what does that mean? Popular folklore would have you believe that performance cams are more aggressive than stock cams. They must be more aggressive, because they have more lift. Wrong, wrong, wrong! Lift has nothing to do with it. You can have a lazy slow opening cam with a lot of lift simply because it has a lot of duration.
Harvey Crane has a
webpage that
discusses the term aggressive. He likes to use a term he
coined called intensity. Intensity is a measure of
how quickly a cam opens and closes. He defines intensity base
on 0.050 gross lift (see What's Wrong
with 0.050 Duration?). We will define it as the difference
between the seat-to-seat duration and the 0.040 net duration.
In other words, it is the number of degrees it takes to get the
valve open from zero to a net lift of 0.040 inches plus the number
of degrees to close it from 0.040 net to zero. So, intensity
is a measure of quick opening (and closing) as we
have discussed it on the
Cam Performance page.
The graph at right is the left half of a typical lift curve with velocity, acceleration and jerk also plotted. The right half of the curve is a mirror image. Intensity is four times the distance between the two vertical bars (x2 for open and close, x2 to get crank degrees). If you look at the lift curve, you could get the valve open quicker if you had more upward curvature. Upward curvature is acceleration. To increase the acceleration quickly, you must have high jerk. You could also get the valve open quicker if the lift curve had a greater slope when the valve opens. The slope at the opening point is the ramp velocity.
This definition of aggressive does not consider the rate of deceleration at the nose. The deceleration rate has a lot to do with the spring force required to prevent cam/lifter separation or valve float (see Valve Springs). The negative acceleration rate should be added to the list of parameters that determine an aggressive profile.
In summary, an aggressive cam is one with large acceleration (both positive and negative), jerk and ramp velocity. An aggressive cam will open and close quickly, i.e. will have low Intensity. Cams with aggressive profiles will generate greater drive train forces (see Valve Train Dynamics), but will give better performance (see Cam Performance)
Contrary to popular beliefs, we have found that performance cams are often less aggressive than their stock counterparts (see Flathead Cams and Triumph Cams).